Catalyst components of the Ziegler-Natta type for the stereospecific polymerization of olefins are widely known in the art. The latest developed catalysts for propylene polymerization comprise a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and an internal electron donor compound, used in combination with an Al-alkyl compound and often with an external donor which is needed in order to obtain higher isotacticity. One of the preferred classes of internal donors is constituted by the esters of phthalic acid, diisobutylphthalate being the most used. The phthalates are used as internal donors in combination with alkylalkoxysilanes as external donor. This catalyst system is capable of giving good performances in terms of activity, and propylene polymers with high isotacticity and xylene insolubility endowed with an intermediate molecular weight distribution.
Use of some phthalates however has been recently addressed as involving potential toxicity problems and therefore research activities have been devoted to find alternative classes of donor capable of replacing phthalates in terms of both performances and quality of the product. U.S. Pat. No. 7,351,778 discloses esters belonging to the formula R′—CO—O-A-CO—OR1 in which R′ and R1 which may be identical or different, can be substituted or unsubstituted hydrocarbyl having 1 to 20 carbon atoms and A is a single bond or bivalent linking group with chain length between two free radicals being 1-10 atoms, wherein said bivalent linking group is selected from the group consisting of aliphatic, alicyclic and aromatic bivalent radicals, and can carry C1-C20 linear or branched substituents; one or more of carbon atoms and/or hydrogen atoms on above-mentioned bivalent linking group and substituents can be replaced by a hetero-atom selected from the group consisting of nitrogen, oxygen, sulfur, silicon, phosphorus, and halogen atom, and two or more said substituents on the linking group can be linked to form saturated or unsaturated monocyclic or polycyclic ring.
Preferred structures are said to be those in which the bivalent linking group A is a C1-C6 saturated hydrocarbyl group optionally substituted with C1-C4 alkyls. However, the only tested structures are β-benzoyloxy esters in which the bivalent linking group A is made of 2 carbon atoms. The level of performances reported in the patent reference is scattered however, it can be understood that, as an average, the level of stereospecificity/activity balance is to be improved particularly in terms of activity level.
Based on this fact it has been very surprising to discover that a specific subclass of γ-benzoyloxy esters showed improved results in terms of activity/stereospecificity balance with respect to the β-benzoyloxy esters of the prior art.